With customers demanding ever increasing quality and long life, automobile manufacturers have, in turn demanded more from the manufacturers of shaft seals. Some seals of the present invention are intended to be used on wheel spindles of automobiles, and to last without replacement for as long as 100,000 miles, while retaining the grease and excluding grit, dirt, corroding liquids, and so on. The seals heretofore made have not had so long a life. Moreover, the seal is required to be installable in a very small space.
Exclusion of deleterious foreign matter is even more important than the retention of the grease, as in certain applications grease may be added to the wheel bearings from time to time; in fact, it is desirable to have some grease flow out from the grease retaining seal lip into the space between the grease-retaining lip and the dirt excluding lip, so as to lubricate both lips. It is not even harmful to have a small amount of grease expelled via the dirt-excluding lip during lubrication of the wheel bearings. However, the amount allowed to pass the grease-retaining lip should be kept very small.
In any event, there tends to be more wear at the dirt-excluding lip; because that lip is quite likely to come into contact with dirt, grit, dust and corroding liquids. It is also desirable to fling the outside foreign matter outwardly in such a way that it tends not to work its way to the dirt-excluding lip, but even with such flinging, there will still be a substantial amount of deleterious foreign matter attempting to pass by the lip over a period of driving an automobile 100,000 miles.
U.S. Pat. Nos. 4,399,998 and 4,344,631 show some venting structure for seal lips, as does U.S. Pat. No. 2,830,832, but none of these vents is suitable for use in the present seal.
It is also important that the seal be unitized, because with a precision product of this kind and of this size, wrong or careless installation can immediately ruin the sealing lips and lead to the manufacturer's having to make good his guarantee on the wheel bearings. Assemblyline workers in automobile plants are often careless, and even though they may try to be careful, separate installation of a seal and then of a dirt flinging member or of a wear sleeve to go upon the shaft tends to produce misalignments which considerably shorten the life of the seal, and may in fact cause its ruin within a few miles of the running of the new car.
There are many unitized seal strucutres, including those shown in U.S. Pat. Nos. 3,021,161; 3,108,851; 3,179,424; 3,685,841; 3,561,770; 4,028,057; and 4,285,526, but few, if any, are applicable to a seal as narrow as that of the present invention. Some of the enumerated patents show dual-lip seals, but none like those of the present invention.
To insure long life at the dirt exclusion seal, a very long lasting seal composition is desirable. It is possible to make a seal having many of the features of the present invention by using only a standard high quality elastomer such as acrylic rubber, butyl rubber, or an ethylene acrylic rubber (e.g., Vamac.RTM., a DuPont.RTM. Reg. Trademark). However, the use of polytetrafluoroethylene, or an equivalent material, at the actual sealing edges is highly desirable. The incorporation of polytetrafluoroethylene becomes important due to the fact that the temperature of the seal during use may rise to between 300.degree. and 350.degree. F. Also the pressure from the grease side of the seal may rise to the range of 50 to 60 p.s.i. at up to about 1200 rpm, which corresponds to vehicle speeds of 80 to 90 miles per hour. Polytetrafluoroethylene also reduces the torque of the seal on the wear sleeve.
Since polytetrafluoroethylene cannot be readily and economically molded to complex shapes, it usually must be incorporated into the seal by using a wafer-like washer, as by bonding it to an elastomeric supporting member. The present invention does this in novel manner employing a novel method of manufacture to produce a novel product. The method further includes features in addition to the incorporation of the polytetrafluoroethylene.
Polytetrafluoroethylene is shown bonded to elastomers in shaft seals in many patents, including U.S. Pat. Nos. 3,495,843 and 4,239,243 and East German Pat. No. 328,815, but, again the structure of the present invention and the method of producing it do not appear to be known.